In the present invention a fluid polymer is cast as a lining on the interior surface of a closed chamber with a nozzle opening, such as a combustion chamber of a ramjet engine or a rocket motor.
The use of various liners between a solid propellant and its external casing is well known to those skilled in propulsion technology. Liners provide improved mechanical bonding of the solid propellant grain to the metal casing and improved thermal insulation between the grain and casing.
Ablative polymeric materials have been known in aerospace technology for many years, and are commonly used to provide such thermal insulation. A class of ablative polymeric material has been termed "motor case insulation," such as boric acid powder-filled polybutadiene-acrylonitrile, and phenolic resins in chopped carbon fabric, or with a variety of fillers such as glass, silica and asbestos. Other known ablative materials are Teflon, reg. tr., epoxy-novolac, and silicone elastomer.
A good discussion of the use of ablative liners in combustion chambers may be found in "Ablative Polymers in Aerospace Technology" by Donald L. Schmidt in Ablative Plastics, G. F. D'Alelio and John A. Parker, eds. (Marcel Dekker Inc. 1971). Elastomeric ablative insulation for combustion chamber is commonly milled into sheets, bonded together and then vulcanized in place. The many bonded joints that result from this process increase potential liner failure. The chamber may also be tape wrapped with a resin-impregnated woven fibrous tape. The tape is commonly laid in cylindrical and conical sections with the aid of large lathes to compact the tape, which is then heat cured and compression molded.
The recent development of relatively fluid, elastomeric-based ablator formulations has made possible the development of casting techniques to replace the fabrication techniques necessary with conventional polymeric material produced as powder or granules.
The advantages of molding a flowable polymer into a liner have been appreciated in the past. For instance, Woodruff et al. disclose, in U.S. Pat. No. 2,971,225, a method of assembling a mold inside a metal article and injecting a plastic between the mold and the article. Prior art methods would not be suitable for use in a combustion chamber, however, due to the number of external mountings for the mold which are allowed by the relative accessability of the interior of the articles generally coated. The interior of a combustion chamber presents something of a "ship in a bottle" problem, the nozzle being the bottle neck.